“The fastest braking possible comes from a triangular wheel arrangement as on the Vigillante and military fighter jets,” Bob explained while looking over his shoulder at me. “You can lock up the brakes on all three wheels, on wet pavement, in a corner and it won’t go into a spin because more than half of the braking takes place behind the center of gravity.”
I’d heard all of this before, and diagrams had been sketched and annotated with mathematical formulas to show me the validity of his claims, but that 90-degree corner was rapidly approaching and Bob showed no inclination to use the brake. In fact, he continued his oratory while gesticulating with the ever-present unlit stogie in his right hand.
John Lehman had encouraged me to try and dump his Harley trike by doing donuts in the parking lot. I didn’t succeed, but at times it threatened to sacrifice its rider to the Coriolis effect. The 3-wheel arrangement, or at least that with a single lead wheel, is inherently stable, but exceed the lateral gravitational force (“G’s”) limit and over it goes. John simply knew that as long as those handlebars were locked I couldn’t accelerate fast enough to exceed the lateral-g limit, but Bob Keyes wanted me to understand why.
The late Bob Keyes was a rocket scientist, a physicist who invented the TriVette in 1974. His last creation, the Vigilante, is 17-feet of fiberglass, Kevlar, carbon fiber, and aluminum honeycomb composites, that was built to test the theoretical limits of three-wheel performance. Despite its classification, the “Vig” doesn’t look like a motorcycle: actually it resembles an elongated trike with a funny car shell. It even performs like a dragster. Powered by a NASCAR Chevy V-8, the “Vig” will do 0-60 in 3.0 seconds and the quarter mile in 9.97 seconds. It tops out at about 256 mph and will shut down a McLaren and outperform a Ferrari F50. He promoted it as the “quickest street-legal vehicle in the world.”
As he explained it to me—all the while diagramming and writing supporting formula on a piece of scrap paper in an unselfconscious reflex–the Vigilante weighs 1,480 and coming off the line with 345 bhp, 15% of the static load is transferred from the front to the existing 80% load on the rear tires and acceleration reaches .95 G’s. The Vig is 204-inches long and the rear wheels 80-inches wide so this works out to a theoretical tip-over limit of 3.27 lateral G’s – or so I was led to believe.
The math for determining vehicle dynamics is well understood, at least by mathematicians. Of course, there are formulas for static conditions (parked) and dynamic ones (movement). When a vehicle accelerates or decelerates in a straight line, the center of gravity changes position as the load is transferred from one axle or another. To this computation is added either a formula for acceleration or braking. These involve aerodynamic drag, rolling resistance, mass of the vehicle, radius or footprint of the tires, engine torque and drive gear ratios or braking force plus the rotational inertia of driveline components and, of course, the angle of the road surface. Lateral forces include calculations for air density, drag coefficient, aerodynamic lift, and velocity. Cornering performance has its own formula. Finally, these all have to be combined as a summary of forces and movement. What the math doesn’t tell a motorcyclist is that you can’t counter-steer a trike and that you can run over your own foot.
The yellow, diamond-shaped sign with a big, black arrow flashed by. The speedometer the needle was approaching 80 and the long, lean machine was still accelerating, and the big man with the cigar was still talking to me over his shoulder, and then, we were through the corner. Through the corner like it never existed. No screaming tires. No pressing force throwing me to the side. Nothing. Naada.
I never have understood the math, although I still have that scrap of paper in my notes. The “physics of three” is something that I learned to take on faith. Despite several requests to drive the Vigillante, I was always told “another day.” For some reason, Bob didn’t want me piloting his $175,000 baby – apparently the math just didn’t work out.